1. Field of the invention
The present invention relates to a system for controlling the pressure at the well head of a well, and is designed to accommodate logging and completion operations in the well through cable mounted tools.
2. Description of the related art
The capital cost of drilling and evaluating a deep well, for example an oil or natural gas well, is extremely high, and for this reason considerable expense is incurred during those time intervals when drilling or production steps must be interrupted to evaluate the formation. Such evaluation is carried out by lowering in the well a logging sonde or logging tool designed to measure physical parameters representative of the earth formation.
When conducting well-logging operations in a well or borehole, it is necessary to raise and lower the logging tool within the borehole by means of a logging cable. One end of the logging cable is attached to the logging tool via a logging tool connector, and the other end of the logging cable is attached to a winch apparatus which may be disposed on either a suitable truck or an offshore skid unit. It is conventional to pass the logging cable over a plurality of sheave wheels disposed between the winch apparatus and the borehole. Typically, there is an upper sheave wheel attached to the derrick structure rising several feet above the well; the upper sheave deviates the cable at an angle of 180 degrees or so. The cable then engages a lower sheave wheel attached to the derrick structure, above the derrick platform. Moreover, the well is equipped at the surface with a relatively complex pressure control system designed to counterbalance the pressure of the fluid present in the well. The fluid can be either a drilling mud in uncompleted wells, or oil or gas in case of a producing well. A typical pressure control system comprises from the surface to the top: (i) a well head; (ii) a blow out preventer; (iii) a device, usually called a "tool riser", allowing to introduce into the well a logging tool attached to the cable; a tool riser is usually made of several sections of tubes; the last tube is provided at its upper end with a tool head catcher; (iv) a device called a "grease seal" comprising successive pipes, usually called "grease pipes"; the top grease pipe comprises at its end a seal associated with a "cable wiper"; the grease pipes have an internal diameter slightly larger than the cable diameter. High viscosity grease is pumped under high pressure in the annulus between the cable and the internal wall of the grease pipes. While the cable is free to move inside the pipe, the pressurized grease acts as an effective seal against well pressure.
The pressure control system and cable guiding device of the prior art, as hereabove described, present several drawbacks.
All the elements above referred to are disposed end-to-end and thus lead to a substantial height. By way of example, the height of the different elements ar of the following order of magnitude: well head: 3 feet; tool riser: 30 feet; grease seal pipes: 12 feet. The total height of the pressure control system is on the order of 45 feet above ground. Furthermore, an additional foot is required between the top of the pressure control system and the upper sheave wheel which itself measures two feet in height. The total clearance from ground to the top of the sheave is usually around 48 feet. This by itself makes the erection, operation and maintenance of the whole structure complicated, especially when using a crane. Furthermore, on offshore drilling units, the height has such a detrimental effect that it can hinder or even prevent the running of logging operations. An offshore unit generally includes a lower platform where are disposed numerous well heads, typically several tens. Each well head is associated with a well susceptible to be operated from the offshore unit. An upper platform supports the operating set-up including the drilling rig and the personnel and functional facilities. The upper platform is made of a solid floor provided with holes above each well. The elevation between the lower platform and the upper platform is generally about 40 feet or less, while the pressure control system and cable guiding device needed for logging operations are about 48-50 feet high, as already stated. It is impossible to reduce the height of the pressure control system without putting in jeopardy the operation of the same. As a matter of fact, the tool riser height is dependent on the logging tool length. Also, the grease pipes must have a minimum length for given grease viscosity, grease pressure, and pipe internal diameter, so as to be able to balance the well pressure. As a result, no logging operation is possible due to the presence of the drilling rig and the associated upper platform. Since removing those is time consuming and very costly, the logging operations are carried out after all the drilling operations are finished and the rig and platform are removed. This situation, as it can be easily understood, severely limits the opportunities to run logging operations in wells on an offshore site.
Another drawback of prior art devices relates to grease expelled during cable movement from the wiper at the top of the vertical grease seal pipes. The expelled grease accumulates on the upper sheave, and eventually spreads across the well platform or on the ground or in the sea, as wind catches it. This situation is damaging in two respects. First, grease spread on the work area constitutes a hazard since it increases substantially the risks of slipping and falling. Also grease flying in the wind might land on clothes and, with a more serious consequence, in the eyes. Second, the expelled grease is a source of pollution when falling on the ground or in the sea. There is no satisfactory means available so far to collect efficiently the grease expelled.
Prior art devices show a further disadvantage. Putting in place and removing the tool riser, the sheave and the grease pipes (called "rig-up" and "rig-down" operations) are time consuming due to the necessity of untwisting the cable or realigning the sheave, since the sheave has a tendency to spin when being picked up. This situation becomes detrimental when successive logging operations have to be run.
Finally, in prior art devices, it is relatively difficult to align the sheave with the pressure control device. Any off-centering of the sheave with respect to the pressure control device creates an additional stress on the same which adds to the pressure stress coming from the well.
According to the above, there is a strong need for pressure control systems and cable guiding devices which overcome the above mentioned drawbacks.